CFE crashes computing upper bounds of some generic function types

[chloestefantsova]

The CFE crashes compiling the following code:

class A<X1 extends B, X2 extends B> {}
class B {}
A<Y1, Y2> Function<Y1 extends B, Y2 extends B>() f1 = <Y1 extends B, Y2 extends B>() => new A<Y1, Y2>();
A<Z2, Z1> Function<Z1 extends B, Z2 extends B>() f2 = <Z1 extends B, Z2 extends B>() => new A<Z2, Z1>();
test(bool b) {
  var x = b ? f1 : f2;
}

The top of the stack is as follows:

Crash when compiling file:///tmp/qwerty2.dart at character offset 271:
type 'StructuralParameterType' is not a subtype of type 'TypeDeclarationType' in type cast

#0      StandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:kernel/src/standard_bounds.dart:871:15)
#1      TypeSchemaStandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:front_end/src/type_inference/standard_bounds.dart:54:18)
#2      StandardBounds._getNullabilityAwareStandardUpperBound (package:kernel/src/standard_bounds.dart:566:12)
#3      StandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:kernel/src/standard_bounds.dart:848:30)
#4      TypeSchemaStandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:front_end/src/type_inference/standard_bounds.dart:54:18)
#5      StandardBounds._getNullabilityAwareStandardUpperBound (package:kernel/src/standard_bounds.dart:566:12)
#6      StandardBounds._getNullabilityAwareFunctionStandardUpperBound (package:kernel/src/standard_bounds.dart:1347:27)
#7      StandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:kernel/src/standard_bounds.dart:710:16)
#8      TypeSchemaStandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:front_end/src/type_inference/standard_bounds.dart:54:18)
#9      StandardBounds._getNullabilityAwareStandardUpperBound (package:kernel/src/standard_bounds.dart:566:12)
#10     StandardBounds.getStandardUpperBound (package:kernel/src/standard_bounds.dart:558:12)
#11     InferenceVisitorImpl.visitConditionalExpression (package:front_end/src/type_inference/inference_visitor.dart:1085:40)
#12     ConditionalExpression.accept1 (package:kernel/ast.dart:6931:9)
#13     InferenceVisitorImpl._inferExpression (package:front_end/src/type_inference/inference_visitor.dart:292:27)
#14     InferenceVisitorImpl.inferExpression (package:front_end/src/type_inference/inference_visitor.dart:326:40)
#15     InferenceVisitorImpl.visitVariableDeclaration (package:front_end/src/type_inference/inference_visitor.dart:9377:11)
#16     VariableDeclaration.accept (package:kernel/ast.dart:10742:43)
#17     InferenceVisitorImpl.inferStatement (package:front_end/src/type_inference/inference_visitor.dart:275:26)
#18     InferenceVisitorImpl._visitStatements (package:front_end/src/type_inference/inference_visitor.dart:924:50)
#19     InferenceVisitorImpl.visitBlock (package:front_end/src/type_inference/inference_visitor.dart:945:31)
#20     Block.accept (package:kernel/ast.dart:9170:43)
#21     InferenceVisitorImpl.inferStatement (package:front_end/src/type_inference/inference_visitor.dart:275:26)
#22     TypeInferrerImpl.inferFunctionBody (package:front_end/src/type_inference/type_inferrer.dart:184:17)
#23     BodyBuilder.finishFunction (package:front_end/src/kernel/body_builder.dart:1254:43)
#24     DietListener.buildFunctionBody (package:front_end/src/source/diet_listener.dart:1329:19)
#25     DietListener.endTopLevelMethod (package:front_end/src/source/diet_listener.dart:380:5)
#26     Parser.parseTopLevelMethod (package:_fe_analyzer_shared/src/parser/parser_impl.dart:3898:14)
#27     Parser.parseTopLevelMemberImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:3630:14)
#28     Parser.parseTopLevelDeclarationImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:627:14)
#29     Parser.parseUnit (package:_fe_analyzer_shared/src/parser/parser_impl.dart:420:15)
#30     SourceLoader.buildBody (package:front_end/src/source/source_loader.dart:1311:12)
<asynchronous suspension>
#31     SourceLoader.buildBodies (package:front_end/src/source/source_loader.dart:810:7)
<asynchronous suspension>
#32     KernelTarget.buildComponent.<anonymous closure> (package:front_end/src/kernel/kernel_target.dart:734:7)
<asynchronous suspension>
#33     withCrashReporting (package:front_end/src/base/crash.dart:138:12)
<asynchronous suspension>
#34     KernelTarget.buildComponent (package:front_end/src/kernel/kernel_target.dart:711:12)
<asynchronous suspension>

[chloestefantsova]

I'm working on a fix at https://dart-review.googlesource.com/c/sdk/+/379800